Strong Signals at N8UR

AM radio reception isn't a problem at my house... we have several local
stations, and though none of them run more than 10kw, they put a mighty
powerful signal into my antennas. This is a 24 hour plot of the power
delivered by my
AMRAD active LF antenna to any unfortunate receiver it's connected to:

The sudden increase in power occurs when the stations shift from daytime
to nighttime operation. Two of the three loudest ones shift to directional
antenna patterns that double the total energy that I see. Actually, the
situation is even worse than this, because this plot (and the spec
analyzer images below) were taken after a power splitter -- the
single-port power is 3dB greater than this.

By the way, this image was taken using an HP 3586C selective voltmeter
which can measure wideband power between 20Hz and 32MHz with a resolution
of 0.01dB.

This strong (and changing) power at the antenna terminals has a real-world
impact. Here's is a phase plot of a LORAN frequency measurement receiver
showing more than 100 nanoseconds of phase shift when the power level at
its antenna changes:

Apparently, the change in power (more than 1 MHz away from the tuned
frequency) changes the operating conditions of the receiver's front
end, and causes this phase sihft. Adding a low pass filter to reduce the
AM broadcast band signal level by more than 50dB solved the problem.

Here is a spectrum analyzer shot of the signals that create virtually all
of this power, in daytime operation:

And here is the nighttime view. Note that some stations go away (they're
daytime-only) but two of the three major spikes get bigger:

These signals are strong enough to cause some real havoc. I am using a
AMRAD low pass filter to protect my low
frequency receivers, and need to build a similar high pass filter for my
HF gear.